1. Field of the Invention
The present invention relates to synchronous transport networks and in particular to a novel method for providing N:M trail/path protection in SDH or SONET networks.
2. Description of the Prior Art
It is known in the art to protect a physical section or span between two network elements (for instance two ADM's or DXC's) by a MSP “1:1” (or “1+1”) mechanism. In MSP 1+1, a signal is transmitted from a first network element to a second network element and the same signal is permanently bridged by using the other (spare) physical line. At the second network element, the best signal is selected and terminated. On the contrary, in MSP 1:1 mechanism, the signal is bridged on the protection line only in case of failure of the working resources.
For protecting a network through a similar mechanism, a plurality of different MSP 1:1 or 1+1 mechanisms should be provided for protecting the corresponding plurality of physical sections (spans between two nodes) in the network. In any case, possible failures of the network elements (for instance of the crossconnection matrix thereof) are not protected through the MSP mechanism.
In order to overcome the above disadvantages, ITU-T G. 841 Recommendation provides for High Order (or Low Order) linear VC-trail protection mechanism, limited to 1+1 or 1:1 scheme. According to VC-trail mechanism, normal traffic is carried over, or selected from, a protection trail instead of a working trail if the working trail fails or if its performance falls below a required level. For these schemes, ITU-T 707 defines the POH bytes to be used and allocated and the bits within the bytes whilst the protocol rules definition is “for further study”.
As far as SONET technology is concerned, Recommendations Bellcore GR-1400 and Telcordia Technologies GR-253 do not define, at path level (STS1, VT), any protection scheme based on protocol exchange and reserve POH bytes (equivalent to those ones considered in SDH) for “future growth”, not defining, then, their use.
Restoration (i.e. network management assisted switch) requires implicitly the use of an OS application in order to be performed. In multi-operator environment, the network is made-up by different domains with different OS applications. Consequently, the restoration of path(s) installed through the whole network becomes very hard to be performed, since requiring a negotiation between operators, so as the compatibility between OS applications. In addition, the time required for traffic re-routing (hundreds of msec) is highly huge with respect to an automatic process handled at NE level (tens of msec).